The air around us contains particles of different sizes and shapes, which together with exhaust gasses and other contaminations contribute to the total air pollution in an area. Some of the particles are man-made and may originate from e.g the burning of fossil fuels in vehicles. Others occur naturally and may originate from volcanoes, dust storms, forest fires etc. Particles within the 5-500 nm size range are classified as ultrafine particles (UFPs). UFPs, like for instance soot particles, are known to be particularly health-hazardous to humans. It has been verified that inhalation of airborne UFPs can result in severe lung injuries due to their deposition in the lungs.
In view of the above, the measuring of the characteristics of UFPs in the air around us is of great importance. Information regarding the characteristics of airborne ultrafine particles (UFPs) may be collected using a UFP measuring device which enables a local detection of airborne particles and involves the measuring of the particle number concentration N, the number-averaged particle diameter dp,av, and the particle size distribution dN(dp)/dln(dp) in the air. Specifically, the incurred health-hazard associated with exposure to UFP air pollution is believed to relate to the UFP length concentration L=N*dp,av. The reason for the latter inference comes from the consideration (see, for example, H. Fissan et. al., Journal of Nanoparticle Research (2007), Vol. 9, pp. 53-59) that the relative health-hazard of inhaled airborne particles is likely to be associated with the particle surface area per unit volume of inhaled air that deposits in the respiratory tract following inhalation. In addition, this deposited particle surface area concentration can be shown to be proportional with the particle length concentration L in the inhaled air when the deposition efficiency of inhaled airborne particles as a function of their diameter in the various regions of the respiratory tract is properly accounted for (International Commission for Radiological Protection, ICRP, 1994).
A prior art UFP sensor disclosed in WO2007/000710 A2 is illustrated in FIG. 1a. The measuring device 10 comprises an air inlet section 11 which is optionally provided with a particle prefilter 12. The UFP sensor further comprises a particle charging section 18 capable of electrically charging airborne particles in the sampled airflow after their entry into the device 10. In addition, the UFP sensor 10 comprises a particle sensing section 13 comprising a Faraday cage arrangement 16, which is electrically insulated from the remainder of the UFP sensor 10, and which is connected via a sensitive current meter 15 to earth potential. Electrically charged particles in the airflow entering the Faraday cage arrangement 16 are captured by an air-permeable filtration medium inside the Faraday cage together with their charge, thereby giving rise to an electrical current Is, which is measurable by the current meter 15, that is equal to the electrical charge deposited per unit time inside the Faraday cage arrangement 16. The magnitude of the current Is has a proportionality to the concentration level of airborne electrically-charged UFPs in the airflow entering the Faraday cage arrangement 16, the proportionality factor being determined by the average electrical charge on the airborne particles. In case particle charging in the charging section 18 is accomplished by diffusion charging, Is is proportional to the particle length concentration L=N*dp,av (M. Adachi et. al., Journal of Aerosol Sci. 16(2), pp. 109-123, 1985).
The UFP sensor in FIG. 1a is further arranged with a particle concentration variation section 17, arranged downstream from the particle charging section 18, which is capable of causing a variation of the concentration of electrically-charged UFPs between a first concentration level and a second concentration level. In FIG. 1a, the concentration variation section 17 is embodied as a parallel-plate section (also referred to as “plate section”) comprising an air conduit 19 formed by parallel-plate electrode surfaces between which an electric potential difference Vp can be applied. An electric potential difference between the electrode plates creates an electric field across the conduit 19. In case no electric field is applied across the conduit, the concentration level of airborne electrically-charged particles exiting the plate section (the first concentration level) will be substantially the same as the concentration level of airborne electrically-charged particles entering the plate section. In case a non-zero electric field is applied between the plates across the conduit, at least part of the airborne electrically-charged particles entering the plate section will be electrostatically precipitated onto one of the electrode surfaces, thereby reducing the concentration level of the airborne electrically-charged particles exiting the plate section to a smaller second concentration level. The concentration level of electrically-charged particles exiting the plate section is subsequently received by the Faraday cage arrangement 16, giving rise to a sensor current Is that is measured by the current meter 15.
As described above, the magnitude of the measured electrical current signal Is is proportional to the concentration level of electrically-charged UFPs in the airflow received by the Faraday cage arrangement 16 and varies when the concentration level of the electrically-charged UFPs varies. In response to an applied particle concentration variation in the course of time, the known sensor 10 determines measurement signals associated with varied particle concentration levels in a serial way during successive time intervals. A set comprising at least two measurement signals corresponding with a set of at least two varied particle concentration levels is required and sufficient for determining the total particle number concentration N and the average particle diameter dp,av. Different sets of measurement signals can be successively determined to follow the evolution of the total particle number concentration and the average particle diameter in the course of time.
For an accurate determination of the total particle number concentration N and the average particle diameter dp,av of airborne particles, the known sensor 10 requires an environment wherein the total concentration of airborne particles and the particle size distribution (i.e. the particle concentration as a function of particle size) should be no more than only a slowly varying function of time, preferably substantially stationary in time. During the time interval required to measure a single set of two serially obtained measurement signals, as required for a single determination of the total particle number concentration and the average particle diameter, the total particle number concentration and the average particle diameter should remain substantially constant.
This time interval cannot be made arbitrarily small because of minimum required demands on the measurement accuracy that normally necessitate signal averaging during at least a minimum period of time. For accurate operation in a non-stationary transient environment, a device is required that can determine the total particle number concentration N and the average particle diameter dp,av of airborne particles also under highly transient conditions wherein the particle concentration level may rapidly change during the course of time. Such circumstances can for instance arise at or near a location where motorized traffic is present.
In the prior art, and as described above, the particle number concentration N and the average diameter dp,av of airborne particles are inferred from a serial measurement of 2 successively recorded sensor signals Is, one signal Is=I1 being measured at a precipitation voltage Vp=0 in the plate section, the other signal Is=I2 being measured at a precipitation voltage Vp=V1 (see FIG. 1b)). Because an applied non-zero Vp=V1 removes at least part of the electrically-charged particles from the airflow passing through the plate section 17, one will normally have I2<I1.
It is instructive to briefly describe the relative accuracy with which N and dp,av can be inferred with the device 10 from the measured signals I1 and I2 under stationary conditions wherein the characteristics of the size distribution of the electrically-charged airborne particles remain substantially constant in the course of time. At an airflow φ (m3/s) through the sensor, relative to a reference airflow φ* through a proportionally differently sized sensor (the sensor size and the airflow φ being related to each other in such a way that the air velocities inside the sensor remain substantially constant and independent of φ), N relates to I1 and I2 according to Eq. 1:
                    N        =                              S            N                    ⁢                                    ϕ              *                        ϕ                    ⁢                      (                                          I                1                            -                              I                2                                      )                    ⁢                                          ⁢                      (                          particles              /                              cm                3                                      )                                              Eq        .                                  ⁢        1            with SN a first proportionality constant. dp,av relates to I1 and I2 according to Eq. 2:
                              d                      p            ,            av                          =                              S            dp                    ⁢                                    I              1                                                      I                1                            -                              I                2                                              ⁢                                          ⁢                      (            nm            )                                              Eq        .                                  ⁢        2            with Sdp a second proportionality constant. Finally, the particle length concentration L relates to only I1 according to Eq. 3:L=Ndp,av=SNSdpI1 ((particles/cm3)·nm)  Eq.3
Under stationary conditions with respect to N and dp,av, the relative inaccuracies ΔN/N and Δdp,av/dp,av can be shown to relate to the measurement inaccuracy ΔIs of the sensor signal Is according to Eqs. 4 and 5, respectively:
                                                        Δ              ⁢                                                          ⁢              N                        N                    =                                                    2                ⁢                                                                  ⁢                                  S                  N                                            N                        ⁢                                          ϕ                *                            ϕ                        ⁢            Δ            ⁢                                                  ⁢                          I              s                                      ⁢                                  ⁢                                                                                                  Δ                    ⁢                                                                                  ⁢                                          d                                              p                        ,                        av                                                                                                  d                                          p                      ,                      av                                                                      =                                                                            Δ                      ⁢                                                                                          ⁢                                              I                        s                                                                                    I                      1                                                        +                                                            2                      ⁢                      Δ                      ⁢                                                                                          ⁢                                              I                        s                                                                                                            I                        1                                            -                                              I                        2                                                                                                                                                                    =                                                                            ϕ                      *                                        ϕ                                    ⁢                                                            Δ                      ⁢                                                                                          ⁢                                              I                        s                                                              N                                    ⁢                                      (                                                                                                                        S                            N                                                    ⁢                                                      S                            dp                                                                                                    d                                                      p                            ,                            av                                                                                              +                                              2                        ⁢                                                                                                  ⁢                                                  S                          N                                                                                      )                                                                                                          Eq        .                                  ⁢        4            
ΔIS is about 1*10−15 A (=1 fA) for the best operational amplifiers that are currently on the market and cannot easily be made smaller because of electronic noise. This circumstance sets a limit to the attainable accuracy of a single determination of N and dp,av. In addition, the relative uncertainties ΔN/N and Δdp,av/dp,av increase at smaller values for N and/or φ. The airflow φ can be increased to reduce the relative uncertainties/inaccuracies but this cannot generally be done without increasing the sensor size. This increase is undesirable because people normally wish the sensor size to remain as small as possible, also from the point of view of cost and portability. Similarly, a reduction in the sensor size will reduce φ, thereby increasing the relative uncertainties ΔN/N and Δdp,av/dp,av. This increases the scatter in the inferred values of N and dp,av as a function of time. As long as the air pollution characteristics in terms of N and dp,av remain substantially constant in time, an improved degree of accuracy and thus reliability can be accomplished, also at a relatively small value for φ, by averaging the outcomes of successively obtained measurements in the course of time. This averaging can be done either with regard to the measured I1 and I2 signals, or with regard to the inferred values N and dp,av from these signals.
The approach of averaging several serially obtained measurements cannot be used when the air pollution characteristics in terms of N and dp,av change in the course of time (i.e. when they become transient). This is particularly so because the sensor signals I1 and I2 are obtained serially in time when the set-up depicted in FIG. 1a is used. When rapid changes in the air pollution characteristics occur, the successively obtained signals I1 and I2 are recorded under different air pollution conditions and can therefore not reliably be combined together in Eqs. 1-3 for inferring N and dp,av, thereby greatly increasing the relative inaccuracies ΔN/N and Δdp,av/dp,av.
It is quite possible that at some stage I1<I2, which gives a nonsense outcome w.r.t. N and dp,av when Eqs. 1-3 are used. A separate averaging of the serially measured currents I1 and I2 over a certain period of time provides no solution to improve upon this situation because this only tends to dampen the observed air pollution transients that one just wants to measure. Strictly speaking, Eqs. 1 and 2 lose their validity when the serially measured sensor signals I1 and I2 are obtained under different (transient) conditions with respect to the particulate air pollution characteristics. Reliable data for N and dp,av, inferred from the signals I1 and I2 according to Eqs. 1-3, can then not anymore be obtained in the course of time.